1. Field of the Invention
The present invention relates generally to a process of production of alumina which is useful as operational material, such as a support of catalyst or sensor and so forth, separation and purification agent, absorption agent, filter medium, filler of gel chromatography and so on. More specifically, the invention relates to a method for controlling the porous structure of alumina and for controlling the specific surface area thereof.
2. Description of the Background Art
As is well known, many of the properties of alumina are significantly variable depending upon specific surface area, when the alumina is used as operational material, separation and purification agent, absorption agent, filter medium, filler of gel chromatography and so on. Therefore, various attempts have been made for controlling the specific surface area of alumina, silica-alumina compositions, etc. in order to obtain desired properties.
As will be naturally appreciated, the specific surface area of alumina is variable depending upon particle size and the porous structure of the alumina particle. Therefore, the specific surface area of alumina can be controlled by controlling primary particle size and/or the number and diameter of voids on the alumina particle. In one approach, alumina particle size is controlled by the pH swing method proposed in "Catalyst Chemical Course (Shokubai Kagaku Koza)=Catalyst Design" published by Catalyst Academy, page 132. In the proposed process, aluminium salt is precipitate in a solution of a mixture of bemite A10(OH) and amorphous aluminium hydroxide. Then, pH is controlled up and down for smelting of amorphous aluminium hydroxide and growing bemite particles. In this method, the obtained alumina is often conterminated by alkaline pH adjusting agent. Furthermore, alumina obtained though such a precipitation process generally has substantially low heat resistance. In the typical case, after high temperature treatment, such as 1000.degree. C. for three hours, the specific surface area will be reduced to be lower than or equal to 70 m.sup.2 /g. Such specific surface area is insufficient. Therefore, alumina produced through the precipitation process cannot be used as a support for a catalyst or sensor which may be subject to substantially high heat. In view of this, the precipitation process in production of alumina is applicable for only limited fields of use.
On the other hand, one of the typical methods for controlling the porous structure of alumina is to use organic compounds. Such methods have been proposed in "J. Catal." Vol. 1, 1962, pages 1, 547 through 563, and "Catalyst" Vol. 1, No. 3, 1978, page 144. In the proposed process, an organic component is solved in solution of aluminium salt to obtain precipitation of alumina or is added to alumina hydrate or dried gel. In general, polyehthylene glycol, polyvinyl alcohol, polyacryl amide, cellulose ether and so forth are used as the organic compounds. The size of void of alumina through the process set forth may range from 20 .ANG. to 100,000 .ANG.. Such wide range variation of the void size may make alumina useless in applications such as a filter medium, separation and purification agent, or absorption agent, which require substantially high selectivity. Similar processes have also been proposed in Japanese Patent First (unexamined) Publications (Tokkai) Showa 60-21802 and 61-68314. In the former Tokkai Showa 60-21802 , there is proposed a process to precipitate alumina from alumine acid natrium solution in the presence of an organic halogen compound. In the later Tokkai Showa 61-68314, there is proposed a process of gelation of metallic alkoxide in the presence of dihydric alcohol or amino alcohol. In the former case, the void size can be made to range between 10 .ANG. and 30 .ANG.. However, an alkaline component will remain in the produced alumina causing a lowering of specific surface area at high temperature. On the other hand, in the later case, void size can be controlled to be less than or equal to 10 .ANG.. However, in this case, void in the size range of 40 .ANG. to 500 .ANG. is also formed. Therefore, in the later case, it can be said that control of void size is not at all complete.
Another approach has been proposed in "Catalyst" Vol. 20, No. 2, 1978, pages 144 to 154. In this process, alumina hydrate is subject treatment by Monohydric alcohol or acid, and a limited specific surface area range, e.g. 197 m.sup.2 /g to 300 m.sup.2 /g or 199 m.sup.2 /g to 230 m.sup.2 /g is used. The alumina thus produced lacks heat resistance and can cause lack of specific surface area at high temperature range.
In order to improve the foregoing shortcomings in the background art in production of alumina, U.S. patent application Ser. No. 07/403,517, filed on Sept. 6, 1989 with a claim to priority on the basis of Japanese Patent Aplication No. 63-221448, filed on Sept. 6, 1988, which has been assigned of the common owners to the present invention, proposes a "PROCESS OF PRODUCTION OF ALUMINA WITH HIGH SPECIFIC SURFACE AREA".